The present invention generally relates to a logarithmic amplifier circuit. More particularly, the present invention relates to a temperature compensation method for trimming a logarithmic amplifier output.
The increasing demand continues for higher performance products, particularly for use in communication and processing applications. With this demand the need for improvements in the components and devices within these products also increases. This need is particularly keen in connection with logarithmic amplifier devices, as are used, for example, to generate an output that is proportional to the logarithm of the ratio of the input currents. Products incorporating logarithmic amplifiers include various devices such as video amplifiers, medical equipment, analytical instruments, radar and infrared devices, data compression devices, and signal processing devices.
It is often desirable that microelectronic devices produce identical results regardless of the temperature at which they operate. Therefore, temperature changes, ideally, would not effect the output of a logarithmic amplifier. However, log amps using bipolar transistors exhibit temperature drift due to the temperature effects of the thermal voltage (xe2x80x9cVTxe2x80x9d). Therefore, uncorrected, the output of a logarithmic amplifier will drift with temperature. To reduce the effects of this temperature drift, prior temperature drift compensating solutions have trimmed a resistor and thereby created a compensating temperature coefficient (xe2x80x9cTCxe2x80x9d).
For example, prior art temperature compensation techniques trim the temperature coefficient of a second resistor so the reciprocal of its TC will drift in the opposite direction of the VT drift. A typical logarithmic amplifier includes a gain resistor (R1) and a drift trimming resistor (R2). The gain resistor may be trimmed to achieve a desired output gain. However, the VT drifts on the order of, for example, 3300 ppm/xc2x0 C. Therefore, a resistor is trimmed to theoretically offset the 3300 ppm/xc2x0 C. drift of the VT. The drift of a device and the residual drift resulting from prior art techniques varies considerably. While present day logarithmic amplifiers are quite accurate, to meet and exceed the likely demand for even higher performance electronic equipment, new temperature compensation trimming techniques are desirable.
The present invention overcomes the disadvantages of the prior art and provides a new temperature compensation trimming technique. In accordance with an exemplary embodiment of the present invention, temperature compensated output is provided in a logarithmic voltage output device by the steps of: measuring the resistance of a first resistor, a second resistor, and a third resistor at a first temperature; measuring again the resistances of the first resistor, the second resistor, and the third resistor at a second temperature; and trimming the drift of the third resistor according to a calculated temperature compensation trim.